This invention relates to a direct-current machine having a permanent magnet field system with auxiliary poles, wherein the field system is composed of permanent magnets and auxiliary poles.
A type of motor having a field system of permanent magnets and auxiliary poles in the form generally known until now is disclosed in Japanese Patent Laid-Open No. 35721/1973, wherein permanent magnets are distributed peripherally inside a cylindrical yoke and set in mutually opposing positions with an armature core and gaps interposed therebetween, and auxiliary poles made of a magnetic material which acts to intensify the magnetomotive force of an armature reaction are provided.
A motor of this kind has a disadvantage in that its permanent magnets are affected by the demagnetizing field of an armature reaction such as to be decreased in magnetic force. Once the permanent magnets are demagnetized, the motor does not work normally and loading can not be started because of lack of torque. It would be possible for the field system to be designed to keep the necessary magnetic force by giving the permanent magnet an increased radial thickness, but this would mean that the motor would have an uneconomically excessive weight and size.